This invention relates to an inkjet printing process for improving the smear, light stability and density of a printed image containing an ink jet ink containing a water-soluble anionic dye and a cationic receiver.
Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Inkjet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant which is molecularly dispersed or solvated by a carrier medium. The carrier medium can be a liquid or a solid at room temperature. A commonly used carrier medium is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier medium. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light-fastness. When water is used as the carrier medium, such inks also generally suffer from poor water-fastness.
An inkjet recording element typically comprises a support having on at least one surface thereof an ink-receiving or image-forming layer. The ink-receiving layer may be a polymer layer which swells to absorb the ink or a porous layer which imbibes the ink via capillary action.
Ink jet prints, prepared by printing onto ink jet recording elements, are subject to environmental degradation. They are especially vulnerable to water smearing, dye bleeding, coalescence and light fade. For example, since ink jet dyes are water-soluble, they can migrate from their location in the image layer when water comes in contact with the receiver after imaging. Highly swellable hydrophilic layers can take an undesirably long time to dry, slowing printing speed, and will dissolve when left in contact with water, destroying printed images. Porous layers speed the absorption of the ink vehicle, but often suffer from insufficient gloss and severe light fade. There is a need to provide an ink jet recording element which overcomes the above deficiencies.
EP A 1 022 383 A1 discloses a treating agent for a sheet surface to be used in ink jet printing. The treating agent is described as being a graft copolymer composed of a backbone polymer and a branch polymer, either of which is a polymer having vinyl alcohol units, and the other of which is a polymer having cationic groups. However, there is a problem with this graft copolymer in that images printed on an image-receiving layer containing this graft copolymer have undesirable smearing when subjected to high humidity conditions.
It is an object of this invention to provide an ink jet printing method using anionic dyes suitable for use in aqueous inks for ink jet printing that will provide images with better smear, light stability and density using certain receiver elements.
This and other objects are achieved in accordance with this invention which relates to an inkjet printing method, comprising the steps of:
A) providing an ink jet printer that is responsive to digital data signals;
B) loading the printer with ink-receptive elements comprising a support having thereon an image-receiving layer comprising a graft copolymer comprising a backbone copolymer and at least one branch copolymer, the backbone copolymer comprising structural units capable of being oxidized by a transition metal catalyst and the branch copolymer comprising cationic units and neutral hydrophilic units;
C) loading the printer with an ink jet ink composition comprising water, a humectant, and a water-soluble anionic dye; and
D) printing on the image-receiving layer using the ink jet ink in response to the digital data signals.
It has been found that use of the above dyes and image-receiving layer provides excellent smear, light stability and density.
Any anionic, water-soluble dye may be used in a composition employed in the method of the invention such as a dye having an anionic group, e.g., a sulfo group or a carboxylic group. The anionic, water-soluble dye may be any acid dye, direct dye or reactive dye listed in the COLOR INDEX but is not limited thereto. Metallized and non-metallized azo dyes may also be used as disclosed in U.S. Pat. No. 5,482,545, the disclosure of which is incorporated herein by reference. Other dyes which may be used are found in EP 802246-A1 and JP 09/202043, the disclosures of which are incorporated herein by reference. In a preferred embodiment, the anionic, water-soluble dye which may be used in the composition employed in the method of the invention is a metallized azo dye, a non-metallized azo dye, a xanthene dye, a metallophthalocyanine dye or a sulfur dye. Mixtures of these dyes may also be used. An example of an anionic dye which may be used in the invention is as follows: 
The dyes described above may be employed in any amount effective for the intended purpose. In general, good results have been obtained when the dye is present in an amount of from about 0.2 to about 5% by weight of the ink jet ink composition, preferably from about 0.3 to about 3% by weight. Dye mixtures may also be used.
As noted above, the graft copolymer useful in the invention comprises a graft copolymer comprising a backbone copolymer and at least one branch copolymer, the backbone polymer comprising structural units capable of being oxidized by a transition metal catalyst and the branch copolymer comprising cationic units and neutral hydrophilic units.
Examples of backbone polymers comprising structural units capable of being oxidized by a transition metal catalyst include substituted or unsubstituted vinyl polymers or cellulosics, preferably poly(vinyl alcohols), poly(N-vinylpyrrolidones), polyamides, or substituted celluloses such as alkyl celluloses, hydroxyalkyl celluloses, etc. The backbone polymer can be a homopolymer or a copolymer. On being oxidized, the backbone polymer presumably forms a free radical site on the backbone that can attack ethylenically unsaturated branching monomers, thus leading to the formation of a grafted copolymer. A preferred transition metal catalyst is a CeIV salt. Particularly preferred backbone polymers useful in the invention are poly(vinyl alcohols), such as those having from about 60 mol % to about 100 mol % hydrolyzed.
The branch copolymer that is grafted onto the backbone polymer comprises both cationic units as well as neutral, hydrophilic units. Monomers used to introduce cationic units into the branch copolymer include those with the following general structure: 
where R is each independently H or a substituted or unsubstituted alkyl group of 1 to about 4 carbon atoms, M is a group containing cationic charge, and X is an anion or a mixture of anions.
Preferred examples of M are: 
where A is either 0 or NH, each R1, independently represents an alkyl, cyclic alkyl, or aryl group having from 1 to about 20 carbon atoms, and n is an integer from 2 to about 12.
Another preferred example of M is: 
where R1 is defined as above. In addition to the substituents indicated, the ring can also contain an alkyl group of from 1 to about 4 carbon atoms, a phenyl group, a benzyl group, or a second, fused ring.
Still another preferred example of M is: 
where R2 represents H or an alkyl, cyclic alkyl, or alkoxy group having from 1 to about 20 carbon atoms. In addition to the substituents indicated, the ring can also contain an alkyl group of from 1 to about 4 carbon atoms, a phenyl group, a benzyl group, a halide group or a second, fused ring.
Still another preferred example of M is: 
where R2 represents H or an alkyl, cyclic alkyl, or alkoxy group having from 1 to about 20 carbon atoms. In addition to the substituents indicated, the ring can also contain an alkyl group of from 1 to about 4 carbon atoms, a phenyl group, a benzyl group, a halide group, or a second, fused ring.
X is an anion or a mixture of anions, such as halide (e.g., chloride or bromide), alkylsulfate (e.g. methylsulfate), alkylsulfonate (e.g. methylsulfonate), or arylsulfonate (e.g. benzenesulfonate or toluenesulfonate). Other anions can be used if desired. Preferred anions are chloride and methylsulfate.
Specific examples of monomers used to introduce cationic units into the branch copolymer include [(2-acryloyloxy)ethyl]trimethylammonium methylsulfate, [(2-methacryloyloxy) ethyl]trimethylammonium methylsulfate, [(2-methacryloyloxy) ethyl]trimethylammonium chloride, [(3-methacrylamido)-propyl ]trimethylammonium chloride, and [2-(acryloyloxy)ethyl](4-benzoylbenzyl) dimethylammonium bromide. Other examples include (vinylbenzyl)trimethylammonium chloride, (vinylbenzyl)dimethyldodecylammonium chloride, or (vinylbenzyl)dimethyloctadecylammonium chloride.
Monomers used to introduce the neutral, hydrophilic units into the branch copolymer include water-soluble or hydrophilic monomers such as acrylamides, methacrylamides, N-vinylpyrrolidone or suitably substituted vinylpyrrolidones, vinyl ethers, e.g., methyl vinyl ether, hydroxyalkyl esters of acrylates or methacrylates, e.g., 2-hydroxyelhyl methacrylate, and other monomers known to those familiar with the art. A preferred monomer is acrylamide.
Specific examples of the graft copolymers that are useful in the invention are listed below. In each case, the poly(vinyl alcohol) was 80% hydrolyzed and had a molecular weight in the range 8000-10,000. Each of the comonomers was grafted in the amount of 10 weight % compared to poly(vinyl alcohol), unless otherwise indicated.
P-1: Poly(vinyl alcohol)-graft-poly(acrylaniide-co-[(2-methacryloyloxy) ethyl]trimethylammonium methylsulfate)
P-2: Poly(vinyl alcohol)-graft-poly(acrylanlide-co-[(2-methacryloyloxy) ethyl]trimethylamrnonium methylsulfate) [20 wt %/20 wt %]
P-3: Poly(vinyl alcohol)-graft-poly(acrylamide-co-[ar-vomu;bemzyl]trimethylammonium chloride)
P-4: Poly(vinyl alcohol)-graft-poly(acrylarrLide-co-[ar-vinylbenzyl]trimethylammonium chloride) [20 wt %/20 wt %]
P-5: Poly(vinyl alcohol)-graft-poly(acrylanide-co-[(2-methacryloyloxy) ethyl]trimethylamrnonium chloride)
P-6: Poly(vinyl alcohol)-graft-poly(acrylamide-co-[(2-methacryloyloxy) ethyl]trimethylamrnonium methylsulfate)
P-7: Poly(vinyl alcohol)-graft-poly(acrylamide-co-[ar-vinylbenzyl]trimethylammonium chloride)
P-8: Poly(vinyl alcohol)-graft-poly(acrylamide-co-[(3-methacrylamido) propyl]trimethylammonium chloride)
P-9: Poly(vinyl alcohol)-graft-poly(acrylairlide-co-[2-(acryloyloxy)ethyl](4-benzoylbenxyl) dimethylammonium bromide)
P-10: Poly(vinyl alcohol)-graft-poly(acrylamide-co-[ar-vinylbenzyl]dimethyloctadecylammonium chloride)
P-11: Poly(vinyl alcohol)-graft-poly(poly[ethylene glycol]methacrylate-co-[(2-methacryloyloxy) ethyl]trimethylamrnonium methylsulfate)
P-12: Poly(vinyl alcohol)-graft-poly(2-[melhacryloyloxy]ethyl phthalate-co-[(2-methacryloyloxy) ethyl]trimethylamrnonium methylsulfate)
P-13: Poly(vinyl alcohol)-graft-poly[(2-melhacryloyloxy)ethyl]diethylamine-co-[(2-methacryloyloxy) ethyl]trimethylammonium methylsulfate)
P-14: Poly(vinyl alcohol)-graft-poly(acrylanide-co-[(2-methacryloyloxy) ethyl]trimethylamrnonium chloride)
P-15: Poly(vinyl alcohol)-graft-poly(acrylanide-co-[(2-acryloyloxy) ethyl]trimethylammonium methylsulfate)
P-16: Poly(vinyl alcohol)-graft-poly(acrylamnide-co-[(2-acryloyloxy) ethyl]trimethylammonium methylsulfate)
P-17: Poly(vinyl alcohol)-graft-poly(poly[ethylene glycol]methacrylate-co-[(2-methacryloyloxy) ethyl]trimethylamrnonium methylsulfate)
In a preferred embodiment, the weight ratio of cationic to neutral hydrophilic monomer is from about 5:1 to about 1:5, preferably from about 2:1 to about 1:2.
In another preferred embodiment, the weight ratio of the backbone copolymer to the grafted copolymer is from about 20:1 to about 1:5, preferably from about 10:1 to about 1:2.
In another preferred embodiment of the invention, the graft copolymer is present in the image-receiving layer can be used in an amount of from 0.2 to about 40 g/m2, preferably from about 0.5 to about 21.5 g/m2.
The graft copolymers used in this invention can be prepared using conventional polymerization techniques including solution polymerization, inverse emulsion polymerization, inverse suspension polymerization, or other techniques known to those familiar with the art. Particularly convenient is aqueous solution polymerization. A No. of compounds can be used to initiate the graft copolymerization, including azo initiators such as 4,4xe2x80x2-azobis(4-cyanovaleric acid) and its salts, 2,2xe2x80x2-azobis(2-methylpropionamidine) dihydrochloride, and so forth. Other initiators include oxidizing compounds such as persulfate salts, possibly in combination with sulfite salts, or transition metal ions. Particularly useful as initiator is ceric ammonium sulfate.
A binder may also be added to the image-receiving layer employed in the invention, such as poly(vinyl alcohol), poly(l-vinyl pyrrolidone), poly(ethyl oxazoline), non-deionized or deicnized Type IV bone gelatin, acid processed ossein gelatin, pig skin gelatin, acetylated gelatin, phthalated gelatin, oxidized gelatin, chitosan, poly(alkylene oxide), sulfonated polyester, partially hydrolyzed poly(vinyl acetate-co-vinyl alcohol), poly(acrylic acid), poly(1-vinylpyrrolidone), poly(sodium styrene sulfonate), poly(2-acrylamido-2-methane sulfonic acid), polyacrylamide or mixtures thereof In a preferred embodiment of the invention, the binder is gelatin or poly(vinyl alcohol). If a binder is used, then the amount can be up to about 50 wt. % of the image-receiving layer.
Latex polymer particles and/or inorganic oxide particles may also be used as the binder in the image-receiving layer to increase the porosity of the layer and improve the dry time. Preferably the latex polymer particles and/or inorganic oxide particles are cationic or neutral. Examples of inorganic oxide particles include barium sulfate, calcium carbonate, clay, silica or alumina, or mixtures thereof.
Particulates may also be used in the image-receiving layer. In that case, the weight % of particulates in the image-receiving layer is from about 80 to about 95%, preferably from about 85 to about 90%.
The pH of the aqueous ink compositions employed in the invention may be adjusted by the addition of organic or inorganic acids or bases. Useful inks may have a preferred pH of from about 2 to 10, depending upon the type of dye being used. Typical inorganic acids include hydrochloric, phosphoric and sulfuric acids. Typical organic acids include, methanesulfonic, acetic and lactic acids. Typical inorganic bases include alkali metal hydroxides and carbonates. Typical organic bases include ammonia, triethanolamine and tetramethylethlenediamine.
A humectant is employed in the ink jet composition employed in the invention to help prevent the ink from drying out or crusting in the orifices of the printhead. Examples of humectants which can be used include polyhydric alcohols, such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol and thioglycol; lower alkyl mono-or di-ethers derived from alkylene glycols, such as ethylene glycol mono-methyl or mono-ethyl ether, diethylene glycol mono-methyl or mono-ethyl ether, propylene glycol mono-methyl or mono-ethyl ether, triethylene glycol mono-methyl or mono-ethyl ether, diethylene glycol di-methyl or di-ethyl ether, and diethylene glycol monobutylether; nitrogen-containing cyclic compounds, such as pyrrolidone, N-methyl-2-pyrrolidone, and 1,3-dimethyl-2-irridazolidinone; and sulfur-containing compounds such as dimethyl sulfoxide and tetramethylene sulfone. A preferred humectant for the composition employed in the invention is diethylene glycol, glycerol, or diethylene glycol monobutylether.
Water-miscible organic solvents may also be added to the aqueous ink employed in the invention to help the ink penetrate the receiving substrate, especially when the substrate is a highly sized paper. Examples of such solvents include alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; ketones or ketoalcohols such as acetone, methyl ethyl ketone and diacetone alcohol; ethers, such as tetrahydrofuran and dioxane; and esters, such as, ethyl lactate, ethylene carbonate and propylene carbonate.
Surfactants may be added to adjust the surface tension of the ink to an appropriate level. The surfactants may be anionic, cationic, amphoteric or nonionic.
A biocide may be added to the composition employed in the invention to suppress the growth of microorganisms such as molds, fungi, etc. in aqueous inks. A preferred biocide for the ink composition employed in the present invention is Proxel(copyright) GXL (Zeneca Specialties Co.) at a final concentration of 0.0001-0.5 wt. %.
A typical ink composition employed in the invention may comprise, for example, the following substituents by weight: colorant (0.05-5%), water (20-95%), a humectant (5-70%), water miscible co-solvents (2-20%), surfactant (0.1-10%), biocide (0.05-5%) and pH control agents (0.1-10%).
Additional additives which may optionally be present in the inkjet ink composition employed in the invention include thickeners, conductivity enhancing agents, anti-kogation agents, drying agents, and defoamers.
The ink jet inks employed in this invention may be employed in ink jet printing wherein liquid ink drops are applied in a controlled fashion to an ink receptive layer substrate, by ejecting ink droplets from a plurality of nozzles or orifices of the print head of an ink jet printer.
The image-recording layer used in the process of the present invention can also contain various known additives, including matting agents such as titanium dioxide, zinc oxide, silica and polymeric beads such as crosslinked poly(methyl methacrylate) or polystyrene beads for the purposes of contributing to the non-blocking characteristics and to control the smudge resistance thereof, surfactants such as non-ionic, hydrocarbon or fluorocarbon surfactants or cationic surfactants, such as quaternary ammonium salts; fluorescent dyes; anti controllers; anti-foaming agents; lubricants; preservatives; viscosity modifiers; dye-fixing agents; waterproofing agents; dispersing agents; UV absorbing agents; mildew-proofing agents; antistatic agents, anti-oxidants, optical brighteners, and the like. A hardener may also be added to the ink-receiving layer if desired.
The support for the ink jet recording element used in the invention can be any of those usually used for ink jet receivers, such as paper, resin-coated paper, polyesters, or microporo us materials such as polyethylene polymer-containing material sold by PPG Industries, Inc., Pittsburgh, Pa. under the trade name of Teslin(copyright), Tyvek(copyright) synthetic paper (DuPont Corp.), and OPPalyte(copyright) films (Mobil Chemical Co.) and other composite films listed in U.S. Pat. No. 5,244,861. Opaque supports include plain paper, coated paper, synthetic paper, photographic paper support, melt-extrusion-coated paper, and laminated paper, such as biaxally oriented support laminates. Biaxally oriented support laminates are described in U.S. Pat. Nos. 5,853,965; 5,866,282; 5,874,205; 5,888,643; 5,888,681; 5,888,683, and 5,888,714, the disclosures of which are hereby incorporated by reference. These biaxally oriented supports include a paper base and a biaxially oriented polyolefin sheet, typically polypropylene, laminated to one or both sides of the paper base. Transparent supports include glass, cellulose derivatives, e.g., a cellulose ester, cellulose triacetate, cellulose diacetate, cellulose acetate propionate, cellulose acetate butyrate; polyesters, such as poly(ethylene terephthalate), poly(ethylene naphthalate), poly(1,4-cyclohexanedimethylene terephthalate), poly(butylene terephthalate), and copolymers thereof; polyimides; polyamides; polycarbonates; polystyrene; polyolefins, such as polyethylene or polypropylene; polysulfones; polyacrylates; polyetherimides; and mixtures thereof. The papers listed above include a broad range of papers, from high end papers, such as photographic paper to low end papers, such as newsprint.
The support used in the invention may have a thickness of from about 50 to about 500 xcexcm, preferably from about 75 to 300 xcexcm. Antioxidants, antistatic agents, plasticizers and other known additives may be incorporated into the support, if desired. In a preferred embodiment, paper is employed.
In order to improve the adhesion of the image-recording layer to the support, the surface of the support may be subjected to a corona-discharge-treatment prior to applying the image-recording layer.
In addition, a subbing layer, such as a layer formed from a halogenated phenol or a partially hydrolyzed vinyl chloride-vinyl acetate copolymer can be applied to the surface of the support to increase adhesion of the image recording layer. If a subbing layer is used, it should have a thickness (i.e., a dry coat thickness) of less than about 2 xcexcm.
The image-recording layer may be present in any amount which is effective for the intended purpose. In general, good results are obtained when it is present in an amount of from about 2 to about 44 g/m2, preferably from about 6 to about 32 g/m2, which corresponds to a dry thickness of about 2 to about 40 xcexcm, preferably about 6 to about 30 xcexcm.
The following examples illustrate the utility of the present invention.